1. Field of the Invention
The invention relates to a process for continuously casting metal strands and, in particular, steel strands by casting the metal into a straight, vertically oriented mold and subsequently withdrawing the straight strand formed in the mold, wherein the straight strand at first is bent in a bending zone along a transition curve into a circular arc shape having a radius, is guided along a circular arc guide having said radius, subsequently is straightened in a final straightening zone along a transition curve and, after this, is extracted via an approximately horizontal straight guide, as well as a continuous casting apparatus for canrying out said process and a process for converting a continuous casting apparatus including a curved mold into a continuous casting apparatus including a straight mold.
2. Prior Art
It is known, for instance, from AT-B-373 518 and AT-B-331 439 to use a straight mold in order to produce a metal strand by the continuous casting method, i.e., a mold whose cavity receiving the liquid metal is designed straight and oriented in a vertical sense. After emergence from the mold, the strand is guided over a guide comprising a roller system installed either in supporting segments or in through going longitudinal carriers and cooled in a manner that the still thin strand skin is sufficiently supported against bulging caused by the ferrostatic pressure. In most cases, the strand below the mold initially is still guided by a straight guide extending over a length so as to impart on the strand a sufficiently solid strand shell due to direct cooling below the mold such that the strand may subsequently be bent in a bending zone in a manner so as to be able to be further conveyed along a circular arc guide following the bending zone and having a predetermined radius. After the circular arc guide follows a final straightening zone, in which the strand is straightened again. In the straight state, the strand is extracted via an approximately horizontally oriented straight guide.
It is, furthermore, known, for instance, from AT-B-249 896 to cast the strand already in a bent shape in a so-called curved mold in order to produce a metal strand by the continuous casting method, avoiding, a bending zone. In such a curved mold, the cavity into which the liquid metal is cast is arc-shaped and, preferably, circular arc-shaped such that the strand emerging from the mold can be supported and conveyed by a circular arc guide immediately upon its emergence. In that case, it will do to provide a final straightening zone at the end of the circular arc guide, in which the strand is straightened again so as to be subsequently extractable from an approximately horizontal straight guide, as described above.
Continuous casting plants comprising curved molds have been built worldwide; yet, such plants involve metallurgical drawbacks, which are to be seen primarily in that impurities and gas bubbles introduced by the casting jet and penetrating deeply into the strand core can no longer ascend to the meniscus as easily as in a straight mold. Besides, the flow ratios in a curved mold are less beneficial such that disturbances in the shell growth are quite likely to occur.
For the above reasons, attempts have been made to convert continuous casting apparatus including curved molds into continuous casting apparatus including straight molds. If, in doing so, the same strand guide is to be used as provided in the original curved mold continuous casting apparatus, one is forced to arrange a bending zone to precede the circular arc-shaped strand guide as well as a straight continuous casting mold in alignment above the same. However, this involves difficulties since the level of the straight mold will be far above the height of the originally provided curved mold. This is in acceptable, eventually requiring not only an enlargement of the casting hall, but also a redevelopment of the overall hall concept with the casting platform, intermediate vessels and ladle carrying tower as well as the ladle supplies etc. having to be adapted to that new level. As a result, the casting hall will have to be completely reorganized, which is accordingly expensive and, on the other hand, necessitates a long standstill of the casting operation.
Another option consists in replacing the circular arc guide with a circular arc guide adapted to the straight mold and consecutive bending zone. This is, however, also very cumbersome and expensive so that the metallurgical drawbacks of a curved mold will have to be put up with in most cases.